An embodiment system for authenticating a user of a robotaxi includes a smart phone configured to request an authentication of the user based on a digital key, an authentication server configured to create a first authentication number, transmit the first authentication number to the smart phone, receive a second authentication number from the robotaxi when the authentication using the digital key fails, and request unlocking of a door to the robotaxi when the first authentication number and the second authentication number match, and the robotaxi configured to receive the second authentication number from the user, transmit the second authentication number to the authentication server, and unlock the door in response to the request from the authentication server.